This invention relates generally to methods and apparatus for the measurement of the moisture content of a granular material, such as sand or other mineral aggregates which may be used for making concrete or in other applications. More particularly, the invention relates to measurement of the moisture content in a granular material wherein such measurement yields an accurate determination of the moisture in a particular batch of the granular material. A sample of the granular material is measured using a fringe electrode capacitance type gauge which is mounted at a physical location where sampling integrity can be visually verified, thus giving increased accuracy with a much simplified system.
In the concrete products manufacturing area, such as ready-mix batch plants and block plants, it is well known that the water content of a cement concrete mixture has a significant effect on the strength and durability of the cement product after hardening. In many applications the water content of the cement mixture must be very accurately known in order to obtain the proper physical characteristics of the resultant concrete mixture.
It has been found that the problem of measuring with sufficient accuracy the amount of water incorporated into a concrete mixture has been less than satisfactory, especially in manufacturing areas where large amounts of the materials used in the concrete mixture are handled. A ready-mix concrete batch plant, for example, may typically have one hundred concrete recipes for different applications and uses. In most of the recipes, sand, one or more rock aggregates, cement, water, fly ash, and other additives are utilized in large amounts. The ratio between the aggregates and other ingredients is varied in a specific predetermined manner to suit the application and weather conditions in which it is to be used. It is standard practice that such concrete recipes are formulated on the basis of a percentage of ingredients based on their dry weight. Problems arise due to the fact that the rock and sand aggregates are stored in open stock piles, thereby being exposed to moisture, and retaining some moisture in surface moisture. The water introduced as surface moisture as sand, gravel and coarse and fine aggregates may be as much as fifty percent of the total requirements of water necessary in the particular recipe. It is thus essential, in order to batch concrete accurately, that the amount of surface moisture in the aggregates or granular materials be determined for each batch.
It is generally agreed that the rock aggregate surface moisture is relatively constant and therefore an accurate estimate of its surface moisture can safely be used. On the other hand, the surface moisture found in the sand to be mixed with the batch can easily vary from two to ten percent within the stock pile dependent upon the conditions of rain, sunshine or other environmental conditions. The surface moisture of the sand must therefore be accurately determined to eliminate any source of error introduced thereby.
In manufacturing plants which produce batches of concrete it is sometimes necessary to handle up to a ton of sand per minute in order to meet the requirements of the concrete mixers. Alternately, if a batcher is feeding ingredients of the concrete into truck mixers, which mix the batches during transit to the job site, the feed must keep pace with which the trucks can be loaded which is also a very rapid operation. As mentioned hereinbefore, the specifications of a particular concrete batch must be determined very accurately, especially the determination of the amount of water found as surface water on the ingredients. With both stationary central plant mixers and transit mixers, it is usual to utilize a conventional moisture gauge which is mounted inside the bin serving a weigh hopper in which the ingredients are distributed. A moisture gauge is positioned in such a way as to sample the flow of the ingredients in the bin. It has been found with such mounting that the attempt to sample the flow is only an estimation, and sample integrity is not maintained and cannot be monitored. There has been no easy way to verify the flow sampling position of the sensing surface inside the bin thereby creating errors which cannot be tolerated. It is estimated that at least fifty percent of the installed moisture gauges in concrete batch plants are installed in non-flowing sections of the bin.
The operator of the plant who is trying to produce good concrete with predictable slump at the job site requires moisture measurement of the granular materials, and especially of the sand in the batch, to a high degree of accuracy. Many previous attempts found in the prior art yield a moisture measurement of the sand in a static section of the bin which is of no value to the operator.
Previously in the prior art, measurements of the moisture content of sand or other granular materials have utilized measurements of the electrical resistivity and the dielectric properties of the material. It has been found with electrical resistivity measurements, that accurate moisture content can only be determined if the packing of the sand is uniform and the moisture is uniformly distributed through the sand. Such conditions generally do not exist, and therefore more accurate measurements are desired. Alternatively, the dielectric measurements are quite sensitive to the total moisture content of the material, and provide a more accurate determination than resistivity measurements.
It is therefore found to be desirable to provide placement of a measuring apparatus such that sampling of the sand used in a batch of concrete, for example, may be visibly monitored and verified. Additionally, it is desired that a measuring apparatus be designed such that its use does not inhibit or add steps to the manufacturing process.